The present invention relates to a stamper which is used to manufacture an optical device for use in a pickup component for an optical disc such as CD (Compact Disc), CD-ROM (Compact Disc Read Only Memory), MD (Mini Disc), LD (Laser Disc) and so forth. The present invention also relates to a manufacturing method for the stamper and an optical device manufactured therewith.
In general, a hologram device used as a pickup component for an optical disc usually has a size of several square millimeter. In order to manufacture a large number of hologram devices at a low cost, a plurality of hologram devices are formed in a batch on a large light-transmitting substrate and separated into individual hologram devices. An extremely fine diffraction grating is precisely formed in the hologram device. Various methods has been employed for forming this diffraction grating, which are for example a manufacturing method of a semiconductor device as shown in FIGS. 9A to 9F, a forming method called the Photo-Polymer method (hereinafter, referred to as the 2P method) as shown in FIGS. 10A to 10C and FIGS. 11A to 11D and so forth.
Hereafter, a manufacturing method of a hologram device is described with reference to FIGS. 9A to 9F, which method utilizes a manufacturing method of a semiconductor device.
First, one surface of a glass substrate 51 is coated with a photosensitive material 52 by a spin coating method or the like as shown in FIGS. 9A and 9B.
Then, a prescribed pattern is formed in the photosensitive material 52 by photolithography as shown in FIG. 9C.
Subsequently, as shown in FIG. 9D, a fine pattern 51a is formed on the glass substrate 51 in an atmosphere of a gas such as CF4, CHF3 or the like by a Reactive Ion Etching (hereafter, referred to as “RIE”) method. At this time, the photosensitive material 52 is processed together with the glass substrate 51. Therefore, a relationship of the etch rate for the glass substrate 51 and the etch rate for the photosensitive material 52 is confirmed beforehand, and a coating thickness of the photosensitive material 52 is determined so that the photosensitive material 52 remains on the glass substrate 51 even after the pattern 51a having a prescribed depth is formed on the glass substrate 51.
Furthermore, as shown in FIG. 9E, the photosensitive material 52 remaining on the glass substrate 51 is removed by using a solvent or by ashing it under an oxygen gas atmosphere.
Thus, a plurality of hologram devices formed on the glass substrate 51 are divided into finally required shapes H1 as shown in FIG. 9F to complete intermediate products.
However, with the manufacturing method shown in FIGS. 9A to 9F, a long time is required for the RIE process, so that manufacturing efficiency is not improved. Moreover, a diffraction grating cannot be formed on both surfaces of the glass substrate 51 at the same time. Therefore, as one of methods for efficiently forming a diffraction grating on both surfaces at a low cost, the 2P methods shown in FIGS. 10A to 10C and FIGS. 11A to 11D have been proposed.
In the 2P method for manufacturing a hologram device shown in FIGS. 10A to 10C, an original plate 61, on which a fine pattern 61a is formed beforehand, is first coated with an ultraviolet ray curable liquid resin 62, as shown in FIG. 10A. Thereafter, a light-transmitting substrate 63 is placed on the original plate 61 via this ultraviolet ray curable liquid resin 62.
Subsequently, as shown in FIG. 10B, the ultraviolet ray curable resin 62 is pressed and sufficiently expanded in a space formed between the light-transmitting substrate 63 and the original plate 61 while pressurized if required.
Furthermore, after the resin is cured by irradiation of ultraviolet rays, the light-transmitting substrate 63 and the original plate 61 are separated as shown in FIG. 10C. A material having better adhesion to the light-transmitting substrate 63 after curing than to the original plate 61 is selected as the ultraviolet ray curable liquid resin 62, or else the adhesion to the light-transmitting substrate 63 is improved beforehand by pretreatment. Thereby, a resin layer 162 is formed on the light-transmitting substrate 63, so that the resin layer 162 has a pattern 162a transferred from the fine pattern 61a of the original plate 61.
In order to form the transferred pattern on both surfaces of a light-transmitting substrate 73 at the same time, as shown in FIG. 11A, the transparent substrate 73 having primer layers 74, 74′ on both surfaces thereof is firstly positioned between upper and lower original plates 71, 71′ via ultraviolet ray curable resins 72, 72′.
Subsequently, as shown in FIG. 11B, the resins 72, 72′ are pressurized by the upper and lower original plates 71, 71′, and then cured by the irradiation of ultraviolet rays UV, as shown in FIG. 11B. Thereafter, the original plates 71, 71′ are separated from the substrate 73.
Next, as shown in FIG. 1C, anti-reflection films 75, 75′ are vapor-deposited on the resins 72, 72′ which have the patterns 172a, 172′a transferred from the fine pattern 71a, 71′a of the original plate 71 on both surfaces of the substrate 73.
Finally, the substrate is divided into required shapes H2 as shown in FIG. 11D.
In this manufacturing method, fine patterns can be simultaneously formed on both surfaces of the substrate 73 by using the substrate 73 and the original plates 71, 71′ sandwiching this substrate which all transmit light. Therefore, a high integration can be realized by imparting a tracking beam generating function to one surface of one hologram device and an optical branch/error signal generating function to the other surface, and also manufacturing efficiency can be improved.
For improvement of the production yield of hologram devices and for reduction of manufacturing costs by the 2P method, it is essential to form an original plate with a favorable fine pattern. This original plate (hereinafter, referred to as “stamper”) is formed by the procedures shown in FIGS. 12 and 13. It is noted that S-a, S-b, S-e, S-g and S-h of FIG. 12 correspond to FIGS. 13A, 13B, 13E, 13G and 13H to show the same steps of this manufacturing method, respectively.
In step S-a, a quartz substrate 81 is washed. In step S-b, then, the surface of the quartz substrate 81 is coated with a resist 86. In step S-c, the quartz substrate 81 is prebaked to remove a solvent in the resist film. In step S-d, a mask having a fine pattern is aligned and allowed to adhere to the quartz substrate 81 and then exposed. In step S-e, subsequently, photosensitized resist portions are removed by development to form a pattern 87. In step S-f, the substrate is postbaked. In step S-g, RIE, which is a kind of dry etching, is performed in a prescribed depth by using the pattern 87 as a mask. In step S-h, the resist pattern is removed by ashing with an oxygen gas or using a remover. In step S-i, washing is performed as finishing.
However, it has been revealed that a defect called a molding burr generated when a hologram device is manufactured by the 2P method using a stamper. Specifically, as shown in FIG. 14C, the molding burr generates as a defect of a broken piece 97 when an ultraviolet ray curable resin 92 is parted after injecting the ultraviolet ray curable resin 92 into a rectangular groove of a stamper 91 shown in FIG. 14A and curing the resin 92 as shown in FIG. 14B. The broken piece 97 does not usually remain on the stamper side due to the adhesion, and its greater portion is attached to the molded component 92 side. Therefore, a problem arises that the defect generation rate is increased and thus mass productivity is lowered in the manufacture of a hologram device.
As an example of measures for preventing such molding burr, there is Japanese Patent Laid-Open Publication No. 2000-231011. It discloses that a side wall of a stamper groove having a fine pattern is sloped against the stamper surface so that the slope angle θ against the stamper surface satisfies the following relational expression (10), where the depth of the groove in the fine pattern is h and the bottom width of the groove is w.Tan−1(2h/w)<θ<80°  (10)
In addition to the slope, Japanese Patent Laid-Open Publication No. 2000-231011 also discloses that bottom corners of the groove are rounded, that an edge portion are rounded, and that the edge portions are chamfered.
It is evident that the above-stated shapes can reduce generation of the molding burr. However, it has been found that if the side wall of the groove having the fine pattern, which is transferred to the surface of the ultraviolet ray curable resin, is too steep, optical characteristics such as diffraction efficiency are adversely affected. Therefore, a slope angle of the side wall of the groove should be strictly controlled in manufacturing.
When the side wall of the groove in the fine pattern is too steep, primary diffraction efficiency is deteriorated by 2% or more than usual.
A usual hologram pattern is divided into two or more regions, each of which has a different pitch. It is a requirement for the usual hologram device that primary diffraction efficiency in each region is almost equal, specifically, that the ratio of primary diffraction efficiency between the regions (hereinafter, referred to as “efficiency ratio”) is 0.9-1.1.
However, when the side wall of the groove is too steep, there occurs a problem that the efficiency ratio is significantly deteriorated and thereby the primary diffraction efficiencies between the regions cannot be made almost equal.
In order to achieve a shape as described above, Japanese Patent Laid-Open Publication No. 2000-231011 discloses the following methods (I)-(V).    (I) Reactive ion etching is performed at a reduced etch rate over a long time.    (II) A blue plate glass is used as a stamper member which easily generates a slope to the groove side wall.    (III) A wet etching step is performed between a developing step and a reactive ion etching step in a manufacturing process of the stamper.    (IV) A wet etching step is performed between the reactive ion etching step and a resist removing step in a manufacturing process of the stamper.    (V) A wet etching step is performed after the resist removing step in a manufacturing process of the stamper.
However, it was found that the methods (I) to (V) had the following problems in mass production.
In (I), in order to reduce the etch rate, incident electric power is made half or lower compared with the usual case. However, such a low incident electric power allows the etched state to be easily unstable. As a result, there may be cases where uniform etching cannot be achieved in an etching chamber and where the etched state varies at each lot of etching. Thus there easily occurs a problem that stampers cannot be manufactured in consistent quality.
In (II), a blue plate glass is used as a stamper member which easily generates a slope to the groove side wall. However, this member does not transmit ultraviolet rays. Therefore, a stamper using the blue plate glass can be applied to only the case where a pattern is formed on one surface of a substrate. As a result, since the pattern cannot be formed on both surfaces of the substrate, mass productivity of the hologram device is deteriorated.
Each of the manufacturing methods (III)-(V) is a wet etching step to be added to a conventional stamper process. In this wet etching, however, it is difficult to etch the whole stamper uniformly. It is also difficult to perform wet etching of the stamper while controlling a concentration, temperature, reaction product, pH and so forth of an etchant. Thus, there occurs a problem that the etched state of each stamper is not uniform.